1. Field of the Invention
The invention relates to methods and devices for cleaning a gaseous fluid of particles present in said fluid, particularly where the fluid is ionized and then filtered.
2. Prior Art
Swedish patent no. A-9604817-8 discloses a device using an active electret filter. An electric field is directed towards the precipitator unit that consists of medium being electrically non-conductive and whose molecules are easily polarized or oriented by an electric field directed towards the precipitator unit. An electric field is created inside the fiber material comprising the precipitator unit. The particles that pass into the filter medium, being first charged by the ionization unit, are attracted by the filter fibers regardless of the charge polarity of any particular fiber and particle, as each fiber and each particle has spaced-apart positive and negative charges thereon.
U.S. Pat. No. 6,364,935, issued Apr. 2, 2002, to the present inventor, discloses and claims an advanced HEPA filter using polypropylene filter media to attract and capture ionized particles from a gaseous stream, as of household or office air. The arrangement normally provides superior filtering performance, but where the incoming fluid stream is very clean it sometimes happens, as the present inventor has discovered, that ions not bound to particles will impinge on the filter media and themselves charge the media fibers, reducing filter performance by the ions"" repelling, rather than attracting, charged particles in the fluid stream.
Coppom U.S. Pat. No. 5,593,476 shows in FIG. 1 a pre-charging grid 32 and an insulated-wire electrode grid 34 upstream of a polypropylene or other filter 36, and a grounded carbon electrode grid 42 downstream of the filter 36. Power source 48 is attached between the grids 32 and 42. FIG. 2 shows a system in which both upstream grids are charged negatively and the downstream grid is charged positively, thus polarizing the filter media, subject to migration of charges back to the grids. Examples of filter efficiency are shown both with and without ionization.
Gibbs U.S. Pat. No. 5,807,425 shows a charged grid 3 located between two filter mats 1 and 2, which may be of polypropylene. Non-conductive screens 8 and 9 outside the filter mats simply hold the mats in place. Charging of the grid 3 polarizes the filter media, for attracting and holding particles in the media due to their natural polaritiesxe2x80x94no ionization is imparted to them upstream of the filter media.
Dudley U.S. Pat. No. 5,906,677 shows a passive, electrostatic xe2x80x9csuperchargingxe2x80x9d screen 10 located downstream of a filter 14. The screen comprises a thin, fine mesh layer 32 of polypropylene that is supported by metal mesh material 36 on either side. The screen 10 is said to remove fine particles not removed by the conventional filter 14. No ionization is provided.
A flat, open grid or mesh of conductive material is positioned on the upstream side of a pleated polypropylene filter medium, as a first element in a replaceable filter box or cartridge used in an ionizing air cleaner. The grid is not insulated from contact with passing air or the filter medium. The grid is grounded to the charging system in a drain form of the invention, or alternatively it is not so grounded, in a shield form. The effect of either form is to avoid polarization of the filter medium by ions in the air stream that are not attached to particles to be removed from the air stream. A 20-30% increase in filter efficiency is seen when the grid is added onto a filter box or cartridge in this arrangement.